An in vivo implantable device can include a power source, such as a battery, that is rechargeable. The battery can be recharged by wirelessly transferring power from a device positioned ex vivo. Recharging the battery often involves good patient compliance—e.g., by the patient operating the ex vivo device on a set schedule or by the patient visiting a medical professional frequently. Frequently recharging implantable device batteries can be burdensome and failure to recharge the battery frequently can result in inadequate treatment.